Why HF Frequency Measurement Isn't Easy

It's a challenge to measure the frequency of received HF signals with
great accuracy. Propagation effects can cause short term frequency shifts
of several tenths of a Hertz. Here are some screenshots of the WWV
carrier on 10 and 15 MHz measured in the early evening (when it was dark
here in Ohio, but still daylight at the WWV transmitter site in Fort
Collins, Colorado). The "average" screens show about 30 minutes of data
collection, and you can see that the signal moved around quite a bit
during that time.

The signal being recorded is the audio beat frequency from a
single-sideband receiver. The receiver output was fed into
Baudline, a spectrum analyzer
program for Linux.